1. Field of the Invention
The invention is directed to decontaminating the filter media utilized in the purification of phosphoric acid produced by the electrothermal method.
2. Related Art
In a thermal process, phosphoric acid is produced from elemental phosphorus by burning the phosphorus in air and hydrating the resulting P.sub.4 O.sub.10 vapor to form the acid. The only impurity of significance is elemental arsenic. In the production of phosphoric acid, the arsenic is converted to arsenic and arsenous acid. In order to produce a phosphoric acid suitable for food uses, these compounds must be removed. This is accomplished by sulfiding the acid using hydrogen sulfide or other sources of hydrogen sulfide such as sodium hydrogen sulfide and sodium sulfide. The precipitated arsenic sulfides are then removed from the phosphoric acid by pressure filtration, which is usually accomplished using diatomaceous earth as a filter aid. The filter cake produced after filtration is composed largely of the filter material such as diatomaceous earth with arsenic sulfide levels of 1-2%. This contaminated filter material cannot be easily disposed of since it is a regulated hazardous waste. It is an object of the invention to remove the arsenic sulfide from the filter material in order to not only decrease the volume of waste to be disposed of but also to render the filter material suitable for reuse.